(1) Field of the Invention
The present invention relates to a manufacturing method of a light-collecting device to be formed as a lens in a solid-state image sensor and the like, the light-collecting device, and to a phase shift mask.
(2) Description of the Related Art
In general, an apparatus which converts an image into an electric signal (this apparatus is referred to as an imaging apparatus) is used for an appliance which electromagnetically records an image such as a digital video recorder, a digital still camera, and a camera-equipped cellular phone which has been rapidly growing in number. In recent years, a charge-coupled device sensor which is a type of a semiconductor device (this is commonly called as a CCD sensor and is referred to as a CCD sensor, hereinafter) and a MOS sensor are used as such imaging apparatuses. The introduction of such sensors has contributed to make the appliance smaller and lower-priced. Each of these imaging apparatuses is made up of fine-pixels respectively including one photodiode that are arranged on a plane. Accordingly, performance of an imaging apparatus is determined depending on the performances of these number of pixels. The particularly significant performances of the imaging apparatus are a capability of converting a fine input image into an electric signal with low noise (i.e. high S/N ratio) and a capability of outputting the input image as a high electric signal (i.e. with high amplification factor). As a method for realizing a high S/N ratio and high amplification factor, a method of improving the S/N ratio and amplification factor of an imaging device in a pixel is generally suggested. Here, the following method is also commonly adopted.
FIG. 1 is a diagram showing a cross-section of a pixel unit in a typical conventional imaging apparatus. The pixel unit includes a pixel 901 and a light-collecting device 903 which is a lens. The pixel 901 includes a photodiode 902, a color filter 904 and the like. The incident light 905 entering the pixel 901 is collected by the light-collecting device 903 and separated into one of red, blue and green light by the color filter 904, and inputted into the photodiode 902. The intensity and density of the incident light 905 entering the photodiode 902 is increased by the light-collecting device 903. Therefore, the high S/N ratio and the improvement in amplification factor can be realized. Here, a problem is that a focal point of the light-collecting device 903 is changed along with the change of the incident angle of the incident light 905, so that the light cannot be collected on the photodiode 902. This is obvious in the case where the pixel 901 is a peripheral pixel in the imaging apparatus. In order to overcome this problem, there is a conventional technology of arranging light-collecting devices 903 in pixels so as to be asymmetrical to each other (e.g. Japanese Laid-Open Patent Application No. 2001-196568). In addition, there is another conventional technology of shifting the position of the light-collecting device 903 to the photodiode 902 in a peripheral pixel of the imaging apparatus. However, these conventional technologies are effective when the incident angle of the incident light 905 is relatively small. However, they are less effective for a larger incident angle.